Oil sealed mechanical rotating vacuum vane pumps have been extensively utilized in the past, both as primary pumps for vacuum loads of 10.sup.-4 -10.sup.-1 mm. of mercury, or as fore pumps, in combination with diffusion pumps for vacuum loads of greater than 10.sup.-4 mm. of mercury. Such pumps usually include several stages, each having a plurality of vanes that cyclically rotate about an axis of a rotor which is eccentrically mounted in a cylindrical bore of a stator. As the vanes turn, they pump gas between inlet and outlet ports that are physically close to each other in the stator but which are as far as possible from each other in the pumped gas flow path. To this end, the rotor and stator are designed so there is a very narrow gap between them in a region bridging the inlet and outlet. This region is sealed to form a dam for the pumped gas by oil supplied to the pump for sealing and lubrication purposes. During the portion of each vane cycle when the vane approaches the outlet, the pressure of pumped gas in the last, low vacuum stage exceeds atmospheric to flow through a check valve in the last stage outlet and bubble through an oil pool to the atmosphere. The relatively high pressure of the pumped gas condenses vaporized gases, generally water vapor. The molecules of the condensed gas are forced through the dam back to the inlet of the stage and through the outlet which has a deleterious effect on the pump operation.
To obviate this problem it has been the practice to provide a gas ballast device in the vicinity of the outlet of the last stage. The ballast introduces air into the last stage, usually through a spring loaded ball or check valve, to prevent condensation of water vapor to assure that the vapor is pumped out of the pump through the oil pool. The check valve is operated and the air is introduced through an orifice having a location such that there is no direct flow path for the air flowing through the orifice to the last stage inlet at any time during the vane cycle. By properly adjusting the tension on the spring loaded ball valve relative to the tension on the exhaust valve, ejection of the pumped gas and the oil out of the pump through the gas ballast orifice is prevented. In response to the pressure at the orifice reaching a maximum value just before and as the pumping vane reaches the orifice, the ball valve closes to prevent the backflow of oil through the gas ballast valve. The ball valve has several disadvantages, relating to cost and clogging of oil in the vicinity of the ball valve. Oil clogs in the vicinity of the ball valve frequently prevent correct operation of the gas ballast device by causing the ball valve to remain open or to prevent opening of the ball valve to defeat the purpose of the gas ballast. If the ball valve remains open, oil migrates up through the gas ballast inlet, preventing proper operation.